One function of a Heating, Ventilation, and Air Conditioning (HVAC) unit is providing conditioned air to cool an enclosed space, usually a building. During this conditioning, a volume of air is drawn into the HVAC unit, conditioned, and discharged into the building. Typical HVAC units can perform both free cooling and mechanical cooling. Free cooling utilizes the cooler outdoor air as some or all of the air volume. Mechanical cooling is the use of mechanical components to reduce the temperature of the air volume.
An evaporator coil is a conventional component in mechanical cooling. The evaporator coil contains a refrigerant. A blower moves air past the evaporator coil, transferring heat from the air to the evaporator coil. During mechanical cooling, the air volume received by the evaporator coil can be called the “supply air” for mechanical cooling.
Free cooling is preferable to mechanical cooling because free cooling uses substantially less energy. During free cooling, the HVAC unit controller performs an economizing function. The economizing function attempts to make the temperature of the air discharged from the HVAC unit (the “discharge air temperature”) equal to a target temperature (the “free cooling target temperature”). The economizing function increases or decreases the amount of outdoor air drawn into the HVAC unit as necessary to achieve the free cooling target temperature.
Free cooling alone may not be enough to satisfy the cooling demands of a building. If this is the case, the HVAC unit can perform free cooling and mechanical cooling together. However, combining free cooling and mechanical cooling creates difficulties with the amount of outdoor air that should be used as the supply air.
Some HVAC unit controllers continue the economizing function when free cooling and mechanical cooling are combined. In practice, these unit controllers do not receive the benefits of the free cooling. Mechanical cooling tends to quickly reduce the discharge air temperature below the free cooling target temperature. As a result, the economizing function stops utilizing any outdoor air, and the HVAC unit effectively uses only mechanical cooling.
Other HVAC unit controllers instead utilize only outdoor air as the supply air when combining free cooling and mechanical cooling. These unit controllers can produce undesirably cold discharge air. For example, the outdoor air may be 50 degrees Fahrenheit, but the discharge air may be below 32 degrees Fahrenheit after being cooled by the evaporator coil. The resulting air delivered from the HVAC unit may be uncomfortable for the occupants of the building because it is too cold.
This undesirably cold air may also create conditions where the evaporator coil freezes. The cold refrigerant inside the coil can chill the outside surfaces of the coil to below the freezing point of the moisture content of the air. The moisture droplets in the air passing over the evaporator coil may then freeze onto the coil. The ice formation blocks off air flow paths through the coil, resulting in reduced air flow into the building. The frozen evaporator coil may cause the air delivered from the HVAC unit to be uncomfortably humid in addition to being uncomfortably cold.
It would be desirable if an HVAC unit could effectively utilize free cooling and mechanical cooling simultaneously without the risk of undesirably cold discharge air. In particular, the evaporator coil should preferably not cool the discharge air below the freezing point of water.